In some cases where a patient is in need of dental restoration, such as a crown restoration, the tooth or teeth which are to be replaced by the dental restoration are so damaged or weak that an implant needs to be arranged in the patient's bone structure to support the dental restoration. The dental restoration can then be secured at the implant using e.g. a retention screw configured for engaging the bore of the implant and holding the dental restoration firmly in place at the implant.
The dental restoration can be considered to comprise an anatomy portion and a sub-gingival portion, where the anatomy portion is arranged supra-gingival and hence is the visible part of the dental restoration and the sub-gingival portion is the part surrounded by the patient's gingiva when the dental restoration is seated in the patient's mouth.
Different types of dental restorations exist which are adapted for being secured in a dental implant, such as screw retained crowns and an abutment based dental restorations comprising an implant engaging abutment and a crown designed to be seated at the abutment.
For a screw retained crown the anatomy portion and the sub-gingival portion are integrated parts of the dental restoration, such that these portions form a coherent structure.
For abutment based dental restorations, the abutment forms the sub-gingival portion and the crown forms the anatomy portion of the dental restoration. The abutment can be a standard predefined abutment but in many cases the dentist prefers to produce a customized abutment which takes into account the specific situation in the patient's mouth. In the case of a customized abutment a virtual abutment can be designed using dental CAD software, while in the case of a standard abutment the virtual abutment can be provided by the abutment provider. Based on the virtual abutment, the physical abutment can be manufactured using CAM techniques such as milling or 3D printing. In the same manner, a virtual crown part can also be designed based on a digital 3D representation of the patient's set of teeth using CAD software. The crown portion of the dental restoration can then be manufactured based on the virtual crown using CAM techniques.
Often the virtual abutment and the virtual crown are designed separately. Initially a gap between the outer surface of the virtual crown and the outer surface of the virtual abutment may thus occur at the margin line of the virtual crown. This gap has to be closed in order to avoid a grove at the outer surface of the manufactured dental restoration. Such a grove will deteriorate the aesthetic appearance of the manufactured dental restoration and further provide a trap for bacteria and the like.
In prior art methods for designing a dental restoration comprising a crown and an abutment where a gap is occurring between an initial shape of the obtained virtual crown and abutment, the virtual crown is reshaped to contact the virtual abutment. These methods have the disadvantage that the virtual crown is deformed thereby making the appearance of the manufactured crown less attractive and potentially causing discomfort to the patient.